Rigid polyurethane foam insulating boards are extensively used in the construction of built up roofs and as sheathing for insulation of side walls of buildings. Such boards are usually faced with asphalt saturated roofing felt, metal foil or other suitable facing materials. It has, however, been difficult with heretofor known insulation of this type to obtain adequate stability to changing moisture and temperature conditions and the consequent variations in pressure between the facing of the insulating board and the overlying roofing membrane. For instance, when roofing membrane is applied to rigid polyurethane board insulation during the construction of a build up roof, hot asphalt is generally mopped onto the facing of the insulating board and the first ply of roofing membrane is then applied immediately after application of the hot asphalt. Moisture in the saturated felt facing volatilizes and causes frothing of the mopping asphalt. This frothing frequently continues after the first ply of roofing membrane is in place and the pressure generated by the trapped vapors causes localized floating or lifting of the roofing membrane. These lifted areas can be the focus for subsequent blister formation during the next several years as the completed roof ages. Even when non-absorbent facings such as metal foil are used problems frequently arise when hot asphalt is mopped onto the facing and thereby heats gases contained within the foam structure. Since vapors cannot escape through the facing, blistering and delamination of the facing from the foam frequently occurs.
Various methods have been tried to alleviate the above difficulties, but none have been totally successful. It is, for instance, common practice to apply a second facing containing a large number of holes to the insulating board immediately prior to mopping with hot asphalt. The hot asphalt then passes through the holes and secures the second facing to the first facing. This technique reduces the area of facing on the board subject to problems of the sort described above due to contact with the hot asphalt, but, of course, cannot eliminate such problems.
It has also been suggested in U.S. Pat. No. 4,136,223 to utilize a perforated facing on the foam with a second facing being utilized so that the foam material itself forms spots of adhesive to secure the second facing to the remainder of the composite board. This should reduce problems due to direct contact of hot asphalt with the primary facing on the foam, but would allow excessive exposure of the foam material to air with the consequent opportunity for replacement of fluorocarbon gases in the foam with air. As is well known in the industry, the thermal conductivity (K value) of insulating boards containing flurocarbon gas is substantially reduced where some means is not provided to prevent free exchange of fluorocarbon gas normally contained within the cells of the foam insulation with air.